1. Field of the Invention
The present invention relates to a reversible method of annealing magnetic films.
2. Description of the Prior Art
Magneto-optic films and/or various devices utilizing same have been described in the following patents, the teaching of which is incorporated herein by reference thereto: U.S. Pat. Nos. 4,497,545; 4,500,176 and 4,500,177, directed to spatial light modulators. U.S. Pat. Nos. 4,625,167 and 4,755,752, directed to crack detection devices; U.S. Pat. No. 4,728,178, directed to switches, modulators; U.S. Pat. No. 4,604,577, directed to magnetic field sensors; and, U.S. Pat. Nos. 3,838,450; 3,989,352 and 4,608,742, directed to memory and recording/modulator devices. Magneto-optic films have also been used in optical isolators, see e.g., Fujitsu Science & Tech. Journal, Vol. 26, No. 26. Such films may also find utility in rotation sensors for brakes.
Small changes of 1-5 Oe in dynamic Hc were measured by Metselaar and Huyberts, J. Phys. Chem. Solids 34, 2257 (1973), in YIG single crystals and polycrystals. The likely defects were oxygen vacancies. Lotgering, J. Phys. Chem. Solids 36, 1183 (1975), investigated permeability (.mu.) changes in Co or Si doped YIG polycrystals and developed a model of domain wall pinning by anisotropic ions. Dixon and Kurtzig, U.S. Pat. No. 3,759,745 (1973), reported changes in 4.pi.Ms with H.sub.2 treatments of bubble films. Milani and Paroli, J. Appl. Phys. 55, 2173 (1984), studied specifically the H.sub.2 diffusion in bulk YIG and films but only reported on optical properties. A later paper by G. Balestrino, S. Lagomarsino and A. Tucciarone, J. Appl. Phys. 59, 424 (1986), described X-ray lattice constant expansions and contractions with H.sub.2 annealed YIG films. A more recent effort by Kappelt et al., SPIE Vol 274, p. 169 (1990), measured only optical anisotropy changes in epitaxially grown films treated with H.sub.2.
3. The Present Invention
Single crystal bismuth containing garnet films are used in modulators, optical isolators, microwave devices and sensors. The high perfection of these films, however, normally results in such films possessing low (&lt;0.5Oe) as grown coercivity (Hc). As described in U.S. patent application Ser. No. 07/789,362, filed Nov. 8, 1991, the disclosure of which is incorporated herein by reference thereto, the introduction of controlled numbers, sizes and distribution of defects serves to increase coercivity and generate a nearly square hysteresis loop, such features offering favorable effects in switching devices. In accordance with the present invention, the coercivity of magneto-optic films such as bismuth garnet films is adjusted by an annealing method which comprises reduction with a reducing agent such as H.sub.2 followed by oxidation using an oxidizing agent such as O.sub.2 and/or O.sub.3 to adjust to required coercivity levels with the sequence of reduction followed by oxidation repeated one or more times to obtain required coercivity. It is also an aspect of the present invention to precede the first reducing treatment with an oxygen annealing step to enhance uniformity of the switching field.
Oxygen anneal to adjust for coercivity is difficult to control and the yield of acceptable wafers is low. The method of the present invention produces an oxygen annealed magnetic film for the first time, wherein the results of excessive oxygen anneal can be reversed, moreover, the present invention allows oxidation anneal to be performed at lower temperature levels, further improving film yield by not only allowing recovery of over-annealed film, but also limiting film failure. In other words, by preceding anneal at oxidation conditions with anneal at reducing conditions the elevated temperatures generally required for oxygen anneal are no longer required and the irreversibility of excess oxygen anneal is eliminated.